A liquid crystal display (LCD) includes a liquid crystal sandwiched between two transparent sheets. The liquid crystal is composed of long, rod-like, polarized molecules, as shown in FIG. 1. In the absence of an electric field.sup.E, the molecules tend to align themselves with each other, as in FIG. 2A. When a field.sup.E is applied, which exceeds E.sub.C, as indicated, the molecules, being electrically polarized, align themselves with the field, as in FIG. 2B. When the field is increased, the molecules further align as in FIG. 2C.
This tendency to align with an electric field can be used to modulate light, as will be explained by reference to FIGS. 3 and 4. FIG. 3A shows a "twisted nemetic cell." ("Nemetic" is one type of liquid crystal. Two others are "cholesteric"and "smectic.") The molecules can be divided into two types, namely, "body" molecules (ie, those located within the body of the liquid, away from the surface) and "surface" molecules (ie, those at the surface of the body and near the faces of the cell).
The surface molecules are adjacent the faces of the cell, such as faces 3 and 6. These faces are specially treated, in a manner known in the art, in order to induce the surface molecules to align in specific directions. That is, in FIG. 3A, the treatment of face 3 induces the surface molecules 9 to align vertically. The treatment of face 6 induces the surface molecules 12 to align horizontally.
In the absence of an electric field (E=0, as indicated), all molecules (body and surface) tend to align with their neighbor molecules. However, because the surface molecules (at faces 3 and 6) already have an alignment imposed by the treated faces, they induce the body molecules to align with them. Further, since the surface molecules at face 3 are perpendicular to those at face 6, the body molecules must make the transition from vertical alignment (at face 3) to horizontal alignment (at face 6). They do this by forming the "twisted" arrangement shown.
When an electric field is applied which greatly exceeds the critical field (ie, E&gt;&gt;E.sub.C, as indicated), the body molecules change alignment; they now align themselves with the field instead of with the surface molecules, as shown in FIG. 3B.
The apparatus FIG. 3 can be used to construct one type of LCD, namely, the "reflective"type, as shown in FIG. 4. Unpolarized incoming radiation, such as interior room light, passes through a polarizing filter 15 and becomes polarized. The direction of polarization is parallel to the molecules at face 3 of the cell, and thus the polarized light is allowed to enter the cell. As the polarized light progresses through the cell, the direction of polarization gradually changes, following the direction of the "twisted" molecules. That is, the "twisted" molecules cause the polarization vector of the light to rotate as the light propagates through the cell. When the polarized light reaches face 6, it has rotated ninety degrees, and its polarization vector is now parallel with the molecules at the second face 6. The light exits the second face 6, passes through a second polarizing filter 18, perpendicular to the first, and is reflected by a mirror 21.
After reflection, the light retraces its steps. It again rotates ninety degrees as it passes through the cell, and exits through the first polarizing filter. Thus, ignoring attenuation occurring within the filters and the cell, the light appears to have been reflected by the mirror 21. The cell looks bright.
If an electric field is applied to the cell the molecules align as shown in FIG. 3B. The "twisting" disappears, removing the ninety-degree rotation of light within the cell. With the rotation absent, the second polarizing filter 18 blocks transmission of the light, because the light reaching it has the wrong polarization for transmission through it. There is no reflection by the mirror 21, and the cell appears dark.
Other types of LCD cells can use the preceding principles, but omit the mirror. These cells use a light source instead of ambient light. These types are called "back-lit."
An actual LCD includes hundreds or thousands of such cells, arranged in a matrix array, as shown by the matrix in FIG. 5. When a high voltage is applied to line 24 and a low voltage is applied to line 27, an electric field, analogous to the field in FIG. 2, is placed across cell 21, and the cell behaves as indicated in FIG. 3B. The electric field causes the cell to appear dark.
One apparatus for applying the electric field is shown in FIG. 6. When a charge is applied to the gate of one (or more) MOS transistors, a current flows through the transistor and charges a capacitor, between whose plates 30 (shown in FIG. 6A) the liquid crystal lies. The charged plates apply an electric field.sup.E to the liquid crystal. The charge on the gate will eventually dissipate, but refreshing circuitry, known in the art, periodically replenishes the gate charge. A common refresh rate is in the range of 50 to 100 Hz.
Another apparatus for applying the electric field is shown in FIG. 7. A pair of diodes in series, but oppositely poled, such as BD11, charges each capacitor, such as CP11. When line 34 is at a high voltage and line 36 is at a low voltage, diode BD11 becomes conducting and charges the capacitor CP11.
(A pair of diodes is used, rather than a single diode, because the voltage on the capacitor must periodically be reversed in polarity. Otherwise, one plate of the capacitor will always be charged from zero to a positive voltage, and tend to permanently align the negative ends of the molecules in a fixed direction. The molecules would thus acquire a "memory," which is undesirable. Periodic reversal of the polarity of the field.sup.E prevents this permanent orientation, and the bi-lateral diodes allow the reversal.)